1. Field of the Invention
The present invention relates to a closed-loop transmitting power control method in a Code Division Multiple Access communication system that controls the transmitting power of a transmitter such that the Eb/I0 value of the signal from each transmitter at a receiver converges on a particular reference value, and in particular, relates to a method of updating the target reference value during closed-loop transmitting power control.
2. Description of the Related Art
The CDMA communication method has received increasing attention in recent years as a communication method for use in a mobile communication systems that is resistant to interference or disturbances. Unlike communication using the FDMA and TDMA methods, which monopolize the transmission channel in terms of frequency or time, communication using the CDMA method features system efficiency that is not limited by the assigned frequency band. In contrast, system efficiency in the CDMA method is determined by the power-to-interference power ratio of the received signal (hereinbelow referred to as xe2x80x9cEb/I0xe2x80x9d). As a result, the power of interference from other channels must be suppressed to the utmost in a mobile communication system that uses the CDMA communication method.
In a communication environment of moving units, differences in propagation loss on transmission lines causes a lack of uniformity in signal level, and raising the system efficiency of a communication realized through by the CDMA method necessitates control of the transmitting power from transmitters such that the received signal level at a receiver is fixed. A closed-loop power control method is one example of a transmitting power control method for achieving this goal. In the closed-loop power control method, the power-to-interference power ratio of the signal received by a receiver is first compared with a target reference value. A signal requesting that the transmitting power be raised if the received signal power-to-interference power ratio is lower than the reference value or that the transmitting power be lowered if the received signal power-to-interference power ratio is greater than the reference value is then inserted in the transmitted signal of that station. The partner station that receives this signal then controls the transmitting power in accordance with the request contained in the signal. If the reference value that is used for comparison is set to a high value in this method, the transmitting power is forced to a high value, giving rise to unnecessary interference between channels. In the interest of raising system efficiency, therefore, the transmitting power must be lowered by setting the reference value to the minimum necessary value within the range that satisfies the requested communication quality.
In an environment of moving units, attenuation due to the length of transmission lines as well as attenuation due to fading fluctuation that accompanies the travel of a mobile station cause the instantaneous received signal level to change with relative rapidity. As a result, the reference value in closed-loop transmitting power control is not a fixed value and must be updated to a value that can prevent degradation in communication quality.
Since a high reference values exert a greater influence on other channels, the reference value is preferably updated so as to converge as rapidly as possibly on the minimum value necessary to satisfy the requested quality. Poor response in updating reference values can cause the problem of degradation in communication quality and breaks in communication. On the other hand, setting the amount of updating too high to guarantee response can lead to extreme degradation of communication quality due to updating of the reference value. In the prior-art method of closed-loop transmitting power control, the amount of updating of the reference value was fixed to a fixed value, with the result that the updating response could not be increased without causing severe degradation of communication quality.
It is an object of the present invention to provide a closed-loop transmitting power control method that can cause the reference value to quickly converge on an optimum value without severely degrading communication quality.
To achieve the above-described object in the closed-loop transmitting power control method of the present invention, a measurement of frame errors during a fixed time interval in the received signal from a transmitter is first carried out at a receiver. If a frame error is not detected in this measurement, a downward updating amount, which is an amount of updating for lowering the reference value, is set according to the magnitude of the reference value, the reference value is lowered by the downward updating amount, and further, an error incidence flag is turned OFF. If a frame error is detected in the frame error measurement, an upward updating amount, which is an amount of updating for raising the reference value, is set according to the logical state of the error incidence flag, the reference value is raised by the upward updating amount, and in addition, the error incidence flag is turned ON.
When lowering the reference value, the present invention determines the amount of downward updating based on the magnitude of the reference value, and can therefore prevent sudden changes in communication quality by making the downward updating amount a small value when the reference amount is small, and can cause the reference value to rapidly converge on the optimum value by making the downward updating amount a large value when the reference value is large. When raising the reference value, the present invention determines the amount of upward updating according to the logical state of the error incidence flag, and thus infers that the received signal level is fluctuating and that communication quality is changing sharply when frame errors occur continuously, whereby the present method can make the upward updating amount a large value to cause the reference value to rapidly converge on the optimum value and thereby suppress degradation in communication quality.
According to another closed-loop transmitting power control method of the present invention, frame errors during a fixed time interval in received signals from a transmitter are first measured at a receiver. If no frame errors are detected in this measurement, the amount of downward updating, which is an amount of updating for lowering the reference value, is set according to the magnitude of the reference value, the reference value is lowered by this downward updating amount, and in addition, the fluctuation level, which is a value indicating the degree of stability of the communication environment, is reduced by xe2x80x9c1xe2x80x9d unless the fluctuation level is xe2x80x9c0.xe2x80x9d If frame errors are detected in the frame error measurement, however, the amount of upward updating, which is an amount of updating for raising the reference value, is set according to the value of the fluctuation level, the reference value is raised by the upward updating amount, and the fluctuation level value is increased by xe2x80x9c1.xe2x80x9d
By determining the downward updating amount based on the magnitude of the reference value when lowering the reference value, the present invention can prevents abrupt changes in communication quality by making the amount of downward updating a small value when the reference value is small, and can cause the reference value to rapidly converge on the optimum value by making the downward updating amount a large value when the reference value is large. Moreover, since the amount of upward updating is determined according to the value of the fluctuation level when raising the reference value, the present invention can infer that the received signal level is fluctuating and that communication quality is changing sharply when frame errors occur continuously, and thus can make the upward updating amount a large level to cause the reference value to rapidly converge on the optimum value, thereby suppressing degradation of the communication quality.